1. Field of the Invention
The present invention relates generally to clips and similar gripping devices, and, more particularly, to a thumbscrew-operated clip for gripping the edges of tarps formed of plastic, cloth or other material.
2. Related Art
The problem of how to secure a tarp against environmental conditions is one of long standing. By their very nature, tarps are intended for use as protection against the weather and are therefore often subjected to high winds. This is true not only in stationary installations, but also where a tarp is used to cover a load on a moving vehicle, such as over a truck bed or rail car.
For years, many tarps have been provided with grommets along their edges to provide attachment points for ropes or other hold-down lines. This adds significantly to the cost of manufacturing the tarp, and unfortunately offers only a partial solution. For example, the grommets sometimes tear out of the edges of the tarp, which can render the tarp useless unless some other means can be found for attaching tie-down lines to its edges. Furthermore, the grommets are ordinarily provided only at widely spaced locations (e.g., at spacing of perhaps three feet or so), which makes it difficult or impossible to attach additional hold-down lines at other points where they may be needed in order to provide a tight fit or to resist wind forces.
Still further, some tarps are not provided with any grommets at all such as VISQUEEN™ and similar plastic sheeting, for example, which makes it extremely difficult to secure these in place. Users have resorted to the expedient of passing ropes or shock (“bungee”) cords over the tops of the sheeting and/or weighting them with bricks, cinder blocks, pieces of wood and similar objects, which is neither secure nor practical in many circumstances.
A number of clip-like attachment devices have been proposed in prior art, principally for use with clothing and woven fabric material. For example, the traditional “suspender clip” uses a pair of metal jaws that are forced together by a clasp mechanism. The sharp, pointed jaws of these devices tend to cause excessive damage and wear to the fabric, and are simply incapable of firmly gripping plastic sheeting or other comparatively thin material without tearing or destroying it. This tendency is complicated by the fact that, due to the nature of the clasp mechanism, this type of clip can only exert a fixed amount of gripping force between the jaws, i.e., the grip cannot be adjusted to be either tighter or looser, as may be needed in particular instances or for use with certain materials. Furthermore, the metal “suspender clip” devices are subject to breakage and rapid wear, and are difficult to operate when wet and cold.
The locking clip disclosed in U.S. Pat. No. 5,388,313 (Cameron) addresses a number of these issues, and is highly effective for many applications. However, the toothed ramp mechanism of this device limits the clamping force to a predetermined range (i.e., between finite upper and lower limits), whereas in some instances it may be desirable to be able to exert a greater or lesser degree of clamping force against the material; for example, when used with certain very thin, slippery or wet materials, it may be desirable to exert a much higher degree of clamping force in order to establish a firmer grip on the material. Furthermore, the teeth on the device shown in the ′313 patent are shown mainly as having the configuration of a series of transversely extending ridges or corrugations; again, while this configuration is very effective for use with many types of materials, other materials may have a tendency to either slip through or tear between the ridged teeth, particularly if forces are applied in a somewhat crosswise direction with respect to the jaws of the clip.
Accordingly, there exists a need for an improved form of clip apparatus which permits an expanded range of grip forces to be exerted against sheet material between the jaws thereof, and which permits a comparatively high grip force to be exerted when desired. Furthermore, there exists a need for such a clip apparatus that is capable of accommodating tarps and materials having a variety of thicknesses. Still further, there exists a need for such a clip apparatus having an arrangement of teeth, which enables the apparatus to establish effective engagement with thin, slick or otherwise hard to grip sheet material. Still further, there exists a need for such a clip apparatus that will minimize damage to the fabric, plastic or other tarp material with which it is used. Still further, there exists a need for such a clip apparatus which is reliable and durable, and which is economical to manufacture.